Electromagnetic Interaction
Electricity and Magnetism

Power supplies for electromagnetism practicals

Teaching Guidance for 11-14 PRACTICAL PHYISCS

Very-low-voltage 'Westminster pattern' supplies are best.

When a wire or coil of thick wire is connected across the terminals of a low-voltage power supply it effectively provides a short circuit.

Most general purpose power supplies are designed so that, under these conditions, an output circuit-breaker operates to switch off the unit. This protects the transformer and rectifiers from damage due to overheating.

The best solution to this problem is to have a special, very-low-voltage power supply for electromagnetism (the Westminster pattern). This has a special transformer with a few, very thick turns as its secondary. The maximum AC voltage is 2 V rms, with a 1 V centre tap. High current rectifier diodes are used to give full-wave rectified DC output at 1 V.

When any of these outputs are shorted, the current flow in the transformer secondary is about 8 A. Most of the power is dissipated in the external circuit, so that the transformer is undamaged. However, the external circuit will become hot.

If a general-purpose power supply is used, even when set to 1 or 2 V AC or DC, the temperature rise in the transformer may damage it or, at best, cause a fuse to operate which can only be replaced by an expert.

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